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This photo shows debris after a blast in Jiangbei District on Nov. 26, 2017, in Ningbo, Zhejiang province of China. The explosion happened around 9 a.m. and caused collapses in the nearby residential area. Two people were found dead and at least 30 were injured in the accident. The reason is under investigation, according to local authorities.

An explosion in a port city south of Shanghai on Sunday killed two people and injured at least 30 others, knocked down buildings and left streets littered with damaged cars and debris, the government and news reports said.

The early morning explosion struck a riverfront neighborhood in Ningbo, one of China's busiest ports, the official Xinhua News Agency and other outlets reported.

Firefighters traced the blast to a hole in the ground where a toilet had been but the cause still was under investigation, state television said on its website. It gave no indication whether the explosion site was inside a building.

Two people were killed and two more seriously injured, the district office announced on its social media account. It gave no details.

Official safety crackdowns have improved conditions in some areas, but many companies still cut corners. In 2015, an explosion traced to improperly stored chemicals killed at least 173 people in Tianjin, a port east of Beijing.

Sunday's blast knocked down residential buildings, but they were vacant and in the process of being demolished, Huanqiu.com said. It said there might have been people in the area collecting scrap for recycling.

Bystanders said the explosion might have been caused by a gas pipeline that was damaged during demolition work, but the Ningbo gas company said it had no lines in the area, the China Youth Daily newspaper reported on its website.

Photos on News.163.com showed an injured woman being carried away on a man's back and what appeared to be the body of man lying in the debris of a wrecked building.

Video clips on multiple websites showed a white cloud of smoke rising above the explosion site and rolling across nearby buildings.

A photo on news.ifeng.com showed a room in an industrial building with a floor-to-ceiling hole blown through an exterior wall. Other photos showed apartments with windows blown out and glass littering the floors.